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Interrupts and Interrupt Controller (INTC)
PXN20 Microcontroller Reference Manual, Rev. 1
10-46 Freescale Semiconductor
10.5.9 Negating an Interrupt Request Outside of its ISR
10.5.9.1 Negating an Interrupt Request as a Side Effect of an ISR
Some peripherals have flag bits that can be cleared as a side effect of servicing a peripheral interrupt
request. For example, reading a specific register can clear the flag bits and consequentially their
corresponding interrupt requests too. This clearing as a side effect of servicing a peripheral interrupt
request can cause the negation of other peripheral interrupt requests besides the peripheral interrupt request
whose ISR presently is executing. This negating of a peripheral interrupt request outside of its ISR can be
a desired effect.
10.5.9.2 Negating Multiple Interrupt Requests in One ISR
An ISR can clear other flag bits besides its own. One reason that an ISR clears multiple flag bits is because
it serviced those flag bits, and therefore the ISRs for these flag bits do not need to be executed.
10.5.9.3 Proper Setting of Interrupt Request Priority
Whether an interrupt request negates outside its own ISR due to the side effect of an ISR execution or the
intentional clearing a flag bit, the priorities of the peripheral or software settable interrupt requests for these
other flag bits must be selected properly. Their PRI
n values in INTC priority select registers
(INTC_PSR0–INTC_PSR315) must be selected to be at or lower than the priority of the ISR that cleared
their flag bits. Otherwise, those flag bits can cause the interrupt request to the processor to assert.
Furthermore, the clearing of these other flag bits also has the same timing relationship to the writing to
INTC end-of-interrupt register (INTC_EOIR_PRC
n ) as the clearing of the flag bit that caused the present
ISR to be executed (see Section 10.4.3.1.2, End of Interrupt Exception Handler ).
A flag bit whose enable bit or mask bit is negating its peripheral interrupt request can be cleared at any
time, regardless of the peripheral interrupt request’s PRI
n value in INTC_PSR x .
10.5.10 Examining LIFO Contents
Normally you do not need to know the contents of the LIFO, or even how deep the LIFO is nested.
Although the LIFO contents are not memory mapped, you can read the contents by popping the LIFO and
reading the PRI field in the INTC current priority register (INTC_CPR_PRC0 or INTC_CPR_PRC1).
Disabling processor recognition of interrupts while examining the LIFO contents provides a coherent view
of the preempted priorities.
The code sequence is:
pop_lifo:
store to INTC_EOIR_PRC n
load INTC_CPR_PRC n , examine PRI, and store onto stack
if PRI is not zero or value when interrupts were enabled, branch to pop_lifo
When you are finished examining the LIFO contents, you can restore it in software vector mode using the
following code sequence:
push_lifo:
load stacked PRI value and store to INTC_CPR_PRC n